Observations of a geomagnetic SI+-SI –pair and associated solar wind fluctuations

Bisoi Susanta Kumar, bisoi.susanta@gmail.com, Physical Research Laboratory, Ahmedabad, India
Padmanabhan Janardhan, jerry@prl.res.in, Physical Research Laboratory, Ahmedabad, India

Abstract
A sudden positive impulse (SI+) in the geomagnetic field (?X) was recorded at all Indian geomagnetic stations at 1835 UT followed by a sudden negative impulse (SI-) at 2300 UT on 23 April 1998. The similar kind of symmetric variation of ?X had been noticed at all mid latitude stations around the world, thus implying the SI+-SI- pair event as a global event. The event was found to be closely correlated to the variations in the solar wind density, while no correspondence with the southward component of magnetic field and solar wind velocity was noticed. The event was associated with a rear-side, partial halo coronal mass ejections (CME) and an M1.4 class solar flare that occurred on the solar west limb on 20 April 1998. This event, to our knowledge, provides the most convincing empirical evidence for the association of specific solar events to the observations of an SI+-SI- pair and shows that a rear-side solar flare can also propagate shock towards the earth.